The oscillations of a crystal are used in a variety of electronic circuits for producing clock signals or other frequency-based signals. Crystal oscillator circuits are commonly used to regulate the oscillations of a crystal. Regulating a crystal oscillator includes applying a controlled amount of voltage across the crystal, wherein a larger voltage applied to the crystal provides greater noise immunity but a shorter crystal life, and conversely, a smaller voltage applied to the crystal provides longer crystal life but a greater amount of noise.
In one conventional approach shown in FIG. 1, regulation of crystal oscillators is achieved by using transistors in a minimalist approach which relies upon the absolute value of transistor thresholds and transconductances (See “High-Performance Crystal Oscillator Circuits: Theory and Application”, Vittoz, E. A.; Degrauwe, M. G. R.; Bitz, S.; Solid State Circuits, IEEE Journal, Volume: 23 Issue: 3, June 1998, Page(s): 774–783).
In another conventional approach shown in FIG. 2, regulation of crystal oscillators is achieved by the application of operational amplifiers for rectification, filtering, and regulation of the oscillator waveform.
In another conventional approach shown in FIG. 3, regulation of crystal oscillators is achieved by the use of external components to set loop bandwidth (See “Analog ALC Crystal Oscillators for High-Temperature Applications”, Raul Andres Bianchi, Jean Michel Karam, and Bernard Courtois; Solid State Circuits, IEEE Journal, Volume: 35 Issue: 1, January 2000, Page(s): 2–14).
Disadvantages of the conventional solutions of FIGS. 2, 3 include the use of one or more operational amplifiers to produce the desired outcome.
As recognized by the present inventors, what is needed is a crystal oscillator control circuit that provides for regulation of the voltage applied to a crystal and reduces or eliminates the need for operational amplifiers.
It is against this background that embodiments of the present invention have been developed.